ASP.net DropDownList with CheckBoxes

I need ti simulate what I am doing on a VB.net Windows form with a ComboBox as per the code below in ASP.net with a DropDownList. Firstly, how would I ensure that the DropDownList has CheckBoxes and secondly what code would I use to do what the following code is doing, Thanks:

You might be able to get similar functionality with the AJAX Control Toolkit's PopUpControl. Just put a CheckBoxList in the PopUpControl.
http://www.asp.net/ajaxlibrary/AjaxControlToolkitSampleSite/PopupControl/PopupControl.aspx

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